The present invention relates to a novel liquid crystal valuable for an electro-optical element utilizing a response of a ferroelectric smectic liquid crystal to an electric field.
Liquid crystals have been utilized as various electro-optical elements and they are used for displaying in watches or electronic table calculators. Liquid crystal elements practically used at the present time are mainly based on the dielectric arrangement effect of a nematic liquid crystal or cholesteric liquid crystal. However, in case of expected application to a display element having many image elements, the response characteristics are insufficient and, since no sufficient driving margin can be maintained, the contrast and visual angle characteristics are insufficient. Accordingly, research and development have been vigorously made on a MOS panel or TFT panel in which a switching element is formed for each image element.
Under these circumstances, Clark et al. developed a liquid crystal element based on a new display principle using the smectic phase, in which the above-mentioned defects of the liquid crystal are eliminated. This liquid crystal element will now be described in brief.
FIG. 1 is a diagram illustrating the smectic C* or H phase. The liquid crystal comprises respective molecule layers 1, and the average direction of the long axis of the molecule in these layers is inclined by an angle .psi..sub.0 relative to the direction vertical to the layers. In the thesis entitled "Ferroelectric Liquid Crystals" in Le Journal de Physique, Vol. 36 (March 1975, pages L-69 to L-71), Meyer et al. teach that a smectic C* or H liquid crystal composed of optically active molecules generally has an electric dipole density P and is ferroelectric. This dipole density P is vertical to the inclination direction n of the molecules and is parallel to the layer face of the smectic phase. Although what is taught by them is applicable also to the smectic H phase, the viscousness to rotation around the axis vertical to the layer is large in the phase H. The presence of an electric dipole in this chiral smectic gives a much stronger coupling force to the electric field than in the dielectric anisotropy. Furthermore, this coupling force has a polar characteristic in the sense that the preferred direction of P is a direction parallel to E. Accordingly, if the direction of the applied electric field is inverted, the direction of P is inverted. Namely, by inversion of the electric field (the angle 2.psi..sub.0 of this cone will be referred to as "cone angle" hereinafter), as shown in FIG. 2, the direction of P can be controlled by the movement of the molecules along the cone. Accordingly, the liquid crystal can be utilized as an electro-optical element by detecting changes of the molecules in the direction of the average long axis by means of two polarizing plates.
In an electro-optical element utilizing the response of this smectic C* or H phase to an electric field, the coupling force between the spontaneous polarization and the electric field is larger on the order of 10.sup.3 to 10.sup.4 than the coupling force by the dielectric anisotropy. Accordingly, the response speed of this element is higher than that of a TN type liquid crystal element and, if appropriate orientation control is selected, a memory characteristic can be given to this element. Therefore, it is expected that this electro-optical element will be applied to a high-speed optical shutter or a display device having a large display information quantity.
Various chiral smectic liquid crystals having this ferroelectric characteristic have heretofore been synthesized and investigated. The ferroelectric liquid crystal first synthesized is p-decyloxybenzilidene-p'-amino.multidot.2-methylbutyl cynnamate generally called "DOBAMBC". Liquid crystals of this series represented by the following structural formula have been synthesized and investigated as ferroelectric liquid crystals: ##STR2## wherein X stands for H, Cl or CN, Y stands for Cl or C.sub.2 H.sub.5, and the asterisk indicates an asymmetric carbon atom.
Since a liquid crystal of this series shows the chiral smectic phase at a relatively high temperature exceeding room temperature, this liquid crystal is defective in that the liquid crystal cannot be used at room temperature. Moreover, since this liquid crystal is of the Schiff base type, it is readily decomposed with water and the stability is poor.
As an improved liquid crystal developed from the above-mentioned series, B. I. Ostrovskii et al. [Ferroelectrics, 24, 309 (1980)] and A. Hallsby et al. [Mol. Cryst. Liq. Cryst., Letter 82, 61 (1982)] proposed a chiral smectic liquid crystal compound of the Schiff base type having a hydroxyl group introduced into one benzene ring and a hydrogen bond in the molecule, as represented by the following general formula: ##STR3## and this compound has attracted attention in the art as a compound showing the smectic C* phase in a broad temperature range including room temperature. Since this compound contains a hydrogen bond in the molecule, it is hardly decomposed with water and is excellent in the stability over ordinary liquid crystals of the Schiff base type. However, this compound is still insufficient because non-crystallization even at temperatures lower than 0.degree. C. is practically required.
An azoxy type liquid crystal material was reported by P. Keller et al. [Ann. Phys., 139 (1978)]. However, since the applicable temperature range is insufficient and this liquid crystal is a densely yellow compound, this liquid crystal material can hardly be put into practical use.
An ester type liquid crystal which is meritorious in the stability among TN type liquid crystal materials is noticeable. In the known literature reference, B. I. Ostrovskii et al. reported that a compound represented by the following formula: ##STR4## is a material showing a chiral smectic liquid crystal phase at temperatures relatively close to room temperature. Moreover, G. W. Gray et al. [Mol. Cryst. Liq. Cryst., 37, 189 (1976) and 48, 37 (1978)] reported a biphenyl ester type material showing a chiral smectic liquid crystal phase at relatively high temperatures.